This command affects output from the theorem prover only when 'prove
output is inhibited; see set-inhibit-output-lst. Calling this macro with
value t as shown above will cause subsequent proof attempts with
'prove output inhibited to print the subgoal number, so that you can see
the progress of the proof; value nil reverts to the default behavior,
where this is not the case. On a related note, we point out that you can
cause output to be saved for later display; see pso and see pso!.